What, if anything, can we make of the fact that, among words beginning with 'th', pronouns and prepositions get the soft sound, and other words the hard? There might be exceptions, but not available to me now.

I'm not sure what you mean by "hard" and "soft" pronunciations of "th" word initially. I used "th*" in the search box on the top page of the alphaDictionary ("Search 1065 Online English Dictionaries at Once!") for common English words beginning with "th." I found three patterns: 1) voiced (IPA ð) as in "this" or "there" 2) unvoiced (IPA θ) as in "think" or "theta" and 3) Indication of aspiration as in "Thailand." I didn't try to count, but I think roots with unvoiced [θ] are more common. (By "root" I want to consider "there" and "therefore" to be variations on the same basic pattern.)

I'd like to re-present this. It seems clear that - among 'th' words - nouns and verbs and most adjectives are pronounced with the 'th' unvoiced. Voiced are prepositions, conjunctions, and - again, help me out with terminology - 'generic' adjectives like 'this' and 'that', and also pronouns. Surely, there's some kind of preference evidenced.Just as in the case of two-syllable words which function as nouns and verbs, ('permit', 'conduct') or nouns and adjectives ('content'). As nouns, they're stressed on the first syllable, and otherwise on the second. To me, the first instance reflects a thinking of nouns as stable, and verbs (adjectives perhaps to a lesser degree) as dynamic, the second syllable stress lending a propulsive quality , if you will. Thinkst thou not?Okay, 'content' is a poor example because the meanings of noun and adjective are not related, but even so . . . .